Set algorithm for phase change memory cell

ABSTRACT

Memory devices and methods for operating such devices are described herein. A method is described herein for operating a memory cell comprising phase change material and programmable to a plurality of resistance states including a high resistance state and a lower resistance state. The method comprises applying a first bias arrangement to the memory cell to establish the lower resistance state, the first bias arrangement comprising a first voltage pulse. The method further comprises determining whether the memory cell is in the lower resistance state, and if the memory cell is not in the lower resistance state then applying a second bias arrangement to the memory cell. The second bias arrangement comprises a second voltage pulse having a pulse height greater than that of the first voltage pulse.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to memory devices based on phase change based memory materials, including chalcogenide based materials and on other programmable resistive materials, and methods for operating such devices.

2. Description of Related Art

Phase change based memory materials, like chalcogenide based materials and similar materials, can be caused to change phase between an amorphous state and a crystalline state by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher electrical resistivity than the generally crystalline state, which can be readily sensed to indicate data. These properties have generated interest in using programmable resistive material to form nonvolatile memory circuits, which can be read and written with random access.

In phase change memory, data is stored by causing transitions in an active region of the phase change material between amorphous and crystalline states. FIG. 1 is a graph of memory cells comprising phase change material and programmable to a plurality of resistance states including a high resistance reset (erased) state 102 and at least one lower resistance programmed (set) state 100, each state having non-overlapping resistance ranges.

The difference between the highest resistance R₁ of the lower resistance state 100 and the lowest resistance R₂ of the high resistance reset state 102 defines a read margin 101 used to distinguish cells in the lower resistance state 100 from those in the high resistance state 102. The data stored in a memory cell can be determined by 5 determining whether the memory cell has a resistance corresponding to the lower resistance state 100 or to the high resistance state 102, for example by measuring whether the resistance of the memory cell is above or below a threshold resistance value R_(SA) 103 within the read margin 101.

The change from the high resistance state 102 to the lower resistance state 10I 100, referred to as set (or program) herein, is generally a lower current operation in which current heats the phase change material above a transition temperature to cause transition from the amorphous to the crystalline state. The change from lower resistance state 100 to the high resistance state 102, referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the phase change process and allowing at least a portion of the phase change material to stabilize in the amorphous state.

In order to reliably distinguish between the high resistance state 102 and the lower resistance state 100, it is important to maintain a relatively large read margin 101. However, since the active region undergoes a phase change as a result of heating, during operation issues such as compositional changes in the phase change material within the active region can result in the formation of a high resistance interface within the electrical conduction path of the memory cell. This high resistance interface can result in a “set failure mode” in which the lower current set operation cannot successfully reduce the resistance of the memory cell below the threshold resistance value R_(SA), resulting in reliability issues and bit errors for those memory cells.

It is therefore desirable to provide a memory device and methods for operating such devices addressing the set failure mode and resulting in improved reliability and improved data storage performance.

SUMMARY OF THE INVENTION

A method is described herein for operating a memory cell comprising phase change material and programmable to a plurality of resistance states including a high resistance state and at least one lower resistance state. The method comprises applying a first bias arrangement to the memory cell to establish the lower resistance state, the first bias arrangement comprising a first voltage pulse. The method further comprises determining whether the memory cell is in the lower resistance state, and if the memory cell is not in the lower resistance state then applying a second bias arrangement to the memory cell to establish the lower resistance state. The second bias arrangement comprises a second voltage pulse having a pulse height greater than that of the first voltage pulse.

A memory device as described herein comprises a memory cell comprising phase change material and programmable to a plurality of resistance states including a high resistance state and a lower resistance state. The memory device further comprises bias circuitry adapted to apply bias arrangements to the memory cell. The bias arrangements include a first bias arrangement to establish the lower resistance state, the first bias arrangement comprising a first voltage pulse. The bias arrangements also include a read bias arrangement to determine whether the memory cell is in the lower resistance state after the set bias arrangement. The bias arrangements further include a second bias arrangement to establish the lower resistance state, the second bias arrangement comprising a second voltage pulse having a pulse height greater than that of the first voltage pulse.

Memory devices and methods for operating such devices described herein address the set failure mode and result in improved endurance, reliability and data storage performance. Set operations described herein comprise applying lower voltages across the phase change material of memory cells to establish the lower resistance state and only applying higher voltages across the phase change material when the lower voltage is insufficient to set the memory cell.

Other aspects and advantages of the present invention can be seen on review of the drawings, the detailed description, and the claims which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of memory cells comprising phase change material and programmable to a plurality of resistance states including a high resistance reset state and at least one lower resistance programmed state.

FIGS. 2A-2C illustrate schematic diagrams of three prior art phase change memory cells having a phase change memory element coupled to a select device.

FIGS. 3A-3E illustrate cross-sectional views of prior art configurations of memory elements.

FIG. 4 is a simplified block diagram of an integrated circuit in which the set operations described herein can be implemented.

FIG. 5 illustrates a portion of the memory array.

FIG. 6 is a flow diagram of a first embodiment of a set operation for programming a memory cell from a higher resistance state to a lower resistance state.

FIG. 7 illustrates an embodiment of a timing diagram of the set operation of FIG. 6.

FIG. 8 illustrates heuristic curves of the temperature versus time for portions of the set operation of FIG. 6.

FIGS. 9A-9C illustrate some alternative pulses for the set bias arrangement of the set operation of FIG. 6.

FIG. 10 is a flow diagram of a second embodiment of a set operation for programming a memory cell from a higher resistance state to a lower resistance state

FIG. 11 illustrates an embodiment of a timing diagram of the set operation of FIG. 10.

FIG. 12 illustrates heuristic curves of the temperature versus time for portions of the set operation of FIG. 10.

FIG. 13 is a flow diagram of an embodiment of a set operation for programming a group of memory cells from a higher resistance state to a lower resistance state.

DETAILED DESCRIPTION

The following description of the disclosure will typically be with reference to specific structural embodiments and methods. It is to be understood that there is no intention to limit the disclosure to the specifically disclosed embodiments and methods, but that the disclosure may be practiced using other features, elements, methods and embodiments. Preferred embodiments are described to illustrate the present disclosure, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows. Like elements in various embodiments are commonly referred to with like reference numerals.

FIGS. 2A-2C illustrate schematic diagrams of three prior art phase change memory cells having a phase change material memory element 220 (represented in the Figures by a variable resistor) and coupled to an access device such as a transistor or diode.

FIG. 2A illustrates a schematic diagram of a prior art memory cell 200 including a field effect transistor (FET) 210 as an access device. A word line 240 extending in a first direction is coupled to the gate of the FET 210 and a memory element 220 couples the drain of the FET 210 to a bit line 230 extending in a second direction.

FIG. 2B illustrates a schematic diagram of memory cell 202 similar to that of FIG. 2A except that the access device is implemented as a bipolar junction transistor (BJT) 212, while FIG. 2C illustrates a schematic diagram of a memory cell 204 similar to that of FIG. 2A except that the access device is implemented as a diode 214.

Reading or writing can be achieved by applying suitable voltages to the word line 240 and bit line 230 to induce a current through the memory element 220. The level and duration of the voltages applied is dependent upon the operation performed, e.g. a reading operation or a writing operation.

FIGS. 3A-3E illustrate cross-sectional views of prior art configurations for memory element 220.

FIG. 3A is a simplified cross-sectional view illustrating a first configuration for memory element 220 coupled to first and second electrodes 312, 314. The first electrode 312 may, for example, be coupled to a terminal of an access device such as a diode or transistor, while the second electrode 314 may be coupled to a bit line.

A dielectric spacer 313 having a width 315 separates the first and second electrodes 312, 314. The phase change material of memory element 220 extends across the dielectric spacer 313 and contacts the first and second electrodes 312, 314, thereby defining an inter-electrode path between the first and second electrodes 312, 314 having a path length defined by the width 315 of the dielectric spacer 313. In operation, as current passes between the first and second electrodes 312, 314 and through the memory element 220, the active region 318 of the phase change material of the memory element 220 heats up more quickly than the remainder of the memory element 220.

FIG. 3B is a simplified cross-sectional view illustrating a second configuration for memory element 220 coupled to first and second electrodes 322, 324. The phase change material of the memory element 220 has an active region 328 and contacts the first and second electrodes 322, 324 at top and bottom surfaces 323, 329 respectively. The memory element 220 has a width 321 the same as that of the first and second electrodes 322, 324.

FIG. 3C is a simplified cross-sectional view illustrating a third configuration for memory element 220 coupled to first and second electrodes 332, 334, the phase change material of memory element 220 having an active region 338. The first and second electrodes 332, 334 are separated by dielectric spacer 335. The first and second electrodes 332, 334 and the dielectric spacer 335 have a sidewall surface 331. The phase change material of memory element 220 is on the sidewall surface 331 and extends across the dielectric spacer 335 to contact the first and second electrodes 332, 334.

FIG. 3D is a simplified cross-sectional view illustrating a fourth configuration for memory element 220 coupled to first and second electrodes 342, 344. The phase change material of memory element 220 has an active region 348 and contacts the first and second electrodes 342, 344 at top and bottom surfaces 343, 349 respectively. The memory element 220 has a width 341 less than that of the first and second electrodes 342, 344.

FIG. 3E is a simplified cross-sectional view illustrating a fifth configuration for memory element 220 coupled to first and second electrodes 354, 352. The first electrode 354 has a width 351 less than width 353 of the second electrode 352 and memory element 220. Because of the difference between width 351 and width 353, in operation the current density in the phase change material of memory element 220 is largest in the region adjacent the first electrode 354, resulting in the active region 358 having a “mushroom” shape as shown in the Figure.

As was described above, in operation issues such as compositional changes in the phase change material within the active region can result in formation of a high resistance interface within the conduction path of the memory cell. The high resistance interface can result in a “set failure mode” in which the resistance of the memory cell cannot be reduced to a resistance corresponding to a lower resistance state using a lower voltage set operation, resulting in reliability issues and bit errors for those memory cells.

FIG. 4 is a simplified block diagram of an integrated circuit 400 in which the set operations (described in more detail below) can be implemented, the set operations addressing the set failure mode and resulting in improved reliability and improved data storage performance of the integrated circuit 400. The integrated circuit 400 includes a memory array 405 implemented using phase change memory cells (not shown). A word line decoder and drivers 410 having read, set, reset, reset verify, set verify, and high-voltage retry modes is coupled to and in electrical communication with a plurality of word lines 415 arranged along rows in the memory array 405. A bit line (column) decoder 420 is in electrical communication with a plurality of bit lines 425 arranged along columns in the array 405 for reading, setting, resetting, reset verify, set verify, and high-voltage retry of the phase change memory cells in array 405. Addresses are supplied on bus 460 to word line decoder and drivers 410 and bit line decoder 420. Sense circuitry (Sense amplifiers) and data-in structures in block 430 are coupled to bit line decoder 420 via data bus 435. Data is supplied via a data-in line 440 from input/output ports on integrated circuit 400, or from other data sources internal or external to integrated circuit 400, to data-in structures in block 430. Other circuitry 465 may be included on integrated circuit 400, such as a general purpose processor or special purpose application circuitry, or a combination of modules providing system-on-a-chip functionality supported by array 405. Data is supplied via a data-out line 445 from the sense amplifiers in block 430 to input/output ports on integrated circuit 400, or to other data destinations internal or external to integrated circuit 400.

The integrated circuit 400 includes a controller 450 for read, set, set verify, reset, reset verify, and high voltage retry modes. The controller 450, implemented in this example using a bias arrangement state machine, controls the application of bias arrangement supply voltages and current sources 455 for the application of bias arrangements including read, set, reset, reset verify, set verify, and high-voltage retry. The controller 450 is coupled to the sense amplifiers in block 430 via feedback bus 475, the controller 450 controlling the bias arrangement supply voltages and current sources 455 in response to output signals of the sense amplifiers. Controller 450 may be implemented using special-purpose logic circuitry as known in the art. In alternative embodiments, controller 450 comprises a general-purpose processor, which may be implemented on the same integrated circuit to execute a computer program to control the operations of the device. In yet other embodiments, a combination of special-purpose logic circuitry and a general-purpose processor may be utilized from implementation of controller 450.

As shown in FIG. 5, each of the memory cells of array 405 includes an access transistor (or other access device such as a diode), four of which are shown as memory cells 530, 532, 534, and 536 having respective phase change memory elements 546, 548, 550, and 552 are illustrated in FIG. 5, representing a small section of an array that can include millions of memory cells. The memory cells are programmable to a plurality of resistance states including a high resistance state and at least one lower resistance state.

Sources of each of the access transistors of memory cells 530, 532, 534, 536 are connected in common to source line 554 that terminates in a source line termination circuit 555, such as a ground terminal. In another embodiment the sources of the access devices are not electrically connected, but independently controllable. The source line termination circuit 555 may include bias circuits such as voltage sources and current sources, and decoding circuits for applying bias arrangements, other than ground, to the source line 554 in some embodiments.

A plurality of word lines 415 including word lines 556, 558 extend in parallel along a first direction. Word lines 556, 558 are in electrical communication with word line decoder 410. The gates of access transistors of memory cells 530, 534 are connected to word line 556, and the gates of access transistors of memory cells 532, 536 are connected to word line 558.

A plurality of bit lines 435 including bit lines 560, 562 extend in parallel in a second direction and are in electrical communication with bit line decoder 420. Memory elements 546, 548 couple the bit line 560 to the respective drains of the access transistors of memory cells 530, 532. Memory elements 550, 552 couple the bit line 562 to the respective drains of the access transistors of memory cells 534, 536.

It will be understood that the memory array 405 is not limited to the array configuration illustrated in FIG. 5, and additional array configurations can also be used. Additionally, instead of MOS transistors, bipolar transistors or diodes may be used as access devices in some embodiments.

In operation each of the memory cells 530, 532, 534, 536 store a data value depending upon the resistance of the corresponding memory elements. The data value may be determined, for example, by comparison of current on a bit line for a selected memory cell to that of a suitable reference current. In a memory cell having three or more states, a plurality of reference currents can be established so that differing ranges of bit line currents correspond to each of the three or more states.

Reading or writing to a memory cell of array 405, therefore, is achieved by applying a suitable voltage to one of word lines 556, 558 and coupling one of bit lines 560, 562 to a voltage so that current flows through the selected memory cell including through the corresponding memory element. For example, a current path 580 through a selected memory cell (in this example memory cell 532 and corresponding memory element 548 are selected) is established by applying voltages to the bit line 560, word line 558, and source line 554 sufficient to turn on the access transistor of memory cell 532 and induce current in path 580 to flow from the bit line 560 to the source line 554, or vice-versa. The level and duration of the voltages applied is dependent upon the operation performed, e.g. a reading operation or a writing operation.

In a reset (or erase) operation of memory cell 532, word line decoder 410 facilitates providing word line 558 with a suitable voltage to turn on the access transistor of the memory cell 532. Bit line decoder 420 facilitates supplying one or more voltage pulses to bit line 560 of suitable amplitude and duration to induce a current to flow though memory element 548, thereby raising the temperature of at least the active region above the transition temperature of the phase change material of the memory element 548 and also above the melting temperature to place at least the active region in a liquid state. The current is then terminated, for example by terminating the voltage pulse on the bit line 560 and the voltage on the word line 558, resulting in a relatively quick quenching time as the active region rapidly cools to stabilize to an amorphous phase. The reset operation can comprise one or more pulses, for example comprising a pair of pulses.

In a read (or sense) operation of memory cell 532, word line decoder 410 facilitates providing word line 558 with a suitable voltage to turn on the access transistor of the memory cell 532. Bit line decoder 420 facilitates supplying a voltage to bit line 560 of suitable amplitude and duration to induce current to flow that does not result in the memory element 448 undergoing a change in resistive state. The current on the bit line 560 and through the memory element 548 is dependent upon the resistance of, and therefore the data state associated with, the memory element 548 of the memory cell 532. Thus, the data state of the memory cell may be determined, for example by comparison of the current on bit line 560 with a suitable reference current by sense amplifiers of sense circuitry 430.

FIG. 6 is a flow diagram of a first embodiment of a set operation 600 for programming memory cell 532 from a higher resistance state to a lower resistance state. FIG. 7 illustrates an embodiment of a timing diagram of the set operation 600 of FIG. 6. As will be understood the timing diagram of FIG. 7 is simplified and not necessarily to scale.

The set operation 600 for selected memory cell 532 begins at step 610. Step 610 may include, or in some embodiments be preceded by, a read operation to determine if the selected memory cell 532 need to be programmed by the set operation 600. The read operation can be accomplished by applying a read bias arrangement such as supplying voltages to word line 558 and bit line 560 sufficient to turn on the access transistor of the selected memory cell 532 and to induce current to flow in path 580 on the bit line 560 and through the memory element 548 to the source line 554 (which is terminated to ground in this example). The current is insufficient for the memory element 548 to undergo a change in resistive state, and the resistance of the memory cell 532 may be determined by comparison of the current on the bit line 560 to a suitable reference current by sense amplifiers of block 430.

Referring to FIGS. 7 and 8, next at step 620 a first bias arrangement is applied to the memory cell 532 to establish the lower resistance state in the memory cell 532. In the illustrated embodiment the first bias arrangement of step 620 comprises applying a voltage V_(WL-SET) to word line 558 above the threshold voltage V_(th) of the access transistor of the selected memory cell 532, and applying a voltage pulse having a pulse height of V_(SET) and pulse length 700 to the bit line 560 to induce current to flow in path 580 and provide a first amount of energy to the phase change material of memory element 548.

As represented heuristically in curve 850 of FIG. 8, the first amount of energy provided to the phase change material of the memory element 548 is sufficient to raise the temperature of at least a portion of the active region of the memory element above the transition (crystallization) temperature 810 of the phase change material. The first amount of energy causes at least a portion of the active region to transition into a crystalline phase, thereby establishing the lower resistance state. As will be understood, the curve 850 is merely illustrative and the actual shape of the curve 850 depends upon the properties of the memory cell, the manner in which the set bias arrangement is applied to the memory cell, and the manner in which the phase change material heats up and cools down.

In the illustrated embodiment of FIGS. 6-8, the first bias arrangement of step 620 comprises a single pulse having a pulse height of V_(SET) and pulse width 700 applied to the bit line 560, although it will be understood that other set bias arrangements can alternatively be used. More generally, a set of one or more pulses may be applied to the bit line 560 and/or word line 558 and/or source line 554 to induce current flow in path 580 to provide the first amount of energy to the phase change material of memory element 548. The number of pulses and the pulse shapes, including the voltage levels and pulse widths, of the first bias arrangement can be determined empirically for each embodiment. FIGS. 9A-9C illustrate some examples of the pulses of the set bias arrangement of step 620 that may be used in some alternative embodiments.

Referring back to FIG. 6, the set method 600 continues to step 630. At step 630 the resistance of the selected memory cell is read to determine whether the memory cell 232 has a resistance corresponding to the lower resistance state. The read operation of step 630 applies a read bias arrangement to the memory cell 232. In the illustrated embodiment the read bias arrangement comprises maintaining the voltage V_(WL-READ) on the word line 558 and applying a voltage pulse having a pulse height of V_(READ) and pulse length 710 to the bit line 560 to induce current to flow in path 580, the current insufficient for the memory element 548 to undergo a change in resistive state. Other read bias arrangements may alternatively be used.

For example, the resistance of the memory cell 532 may be determined by comparison of the current on the bit line 560 to a suitable reference current by sense amplifiers of block 430. Based on the comparison, an output signal of the sense amplifiers of block 430 indicating whether the memory cell has a resistance corresponding to the lower resistance state is supplied to the controller 450 via feedback bus 475. In response to the output signal, the controller 450 terminates the set operation at step 650 if the selected memory cell 538 has a resistance corresponding to the lower resistance state. Other techniques for terminating the set operation may alternatively be used.

If the resistance of the memory cell 532 is not in the lower resistance state, the memory cell 532 has experienced a set failure. This failure may come from the formation of a high resistance layer (or interface) within the electrical conduction path of the memory cell. A retry (or second) bias arrangement is then applied to the memory cell to create the conduction path and establish the lower resistance state, the retry bias arrangement comprising a second voltage pulse across the phase change material having a pulse height greater than the pulse height V_(SET) of the set bias arrangement of step 620.

In the illustrated embodiment of FIG. 6, the retry bias arrangement of the set operation 600 begins at step 640 where a subsequent higher bit line voltage bias arrangement is applied to the memory cell.

Referring to FIGS. 7 and 8, in the illustrated embodiment the subsequent bias arrangement of step 640 comprises applying a voltage V_(WL-RETRY) to word line 558, and applying a voltage pulse having a pulse height of V_(HIGH) and pulse length 720 to the bit line 560 to induce current to flow in path 580 and provide energy to the phase change material of memory element 548. As can be seen in FIG. 7, in the illustrated embodiment the pulse of step 640 has a pulse width less than that of the pulse of step 620 and has a pulse height greater than that of the pulse of step 620.

In embodiments the above mentioned V_(WL-SET), V_(WL-READ), V_(WL-RETRY) can be equal or different. For a typical set-up, using a higher V_(WL-READ) will increase the accuracy of the read operation, while a lower V_(WL-SET) and V_(WL-RETRY) will prevent large current flow through the memory device when doing the set and retry operations.

As represented heuristically in curve 860 of FIG. 8, the pulse having a pulse height V_(HIGH) across the phase change material of the memory element 548 is sufficient to breakthrough the high resistive layer and create a conduction path. As will be understood, the curve 860 is merely illustrative and the actual shape of the curve 860 depends upon the properties of the memory cell, the manner in which the subsequent bias arrangement is applied to the memory cell, and the manner in which the phase change material heats up and cools down.

In some embodiments the bias arrangement of step 640 is the same as the reset bias arrangement used for resetting the memory cell and is sufficient to melt the active region and cause a transition to the high resistance state. Additionally, in some embodiments the subsequent bias arrangement may be current limited, for example by using a lower V_(WL-RETRY), which may prevent damage of the memory device under the high bias condition once breakthrough of the high resistance layer occurs. In FIG. 7 V_(WL-RETRY) is less than V_(WL-SET). As a result, in some embodiments the current induced through the phase change material of the memory element during step 640 can be less than the current induced through the phase change material during step 620.

In the illustrated embodiment of FIGS. 6-8, the subsequent bias arrangement of step 640 comprises a single pulse having a pulse height of V_(HIGH) and pulse width 720 applied to the bit line 560, although it will be understood that other subsequent bias arrangements can alternatively be used. More generally, the subsequent bias arrangement of step 640 may comprise a set of one or more pulses applied to the bit line 560 and/or word line 558 and/or source line 554 to induce current flow in path 580. The number of pulses and the pulse shapes, including the voltage levels and pulse width, of the subsequent bias arrangement can be determined empirically for each embodiment.

In the illustrated embodiment of FIGS. 6-8, the word line voltages for the different steps, including steps 630, 630, and 640 can have different values.

Next, the retry (or second) bias arrangement of the set operation 600 continues back to step 620 where the set bias arrangement is applied to the memory 532.

The set operation 600 then continues to step 630 to determine whether the memory cell 232 has a resistance corresponding to the lower resistance state. The set operation continues in the loop of iteratively applying the retry (or second) bias arrangement (steps 640, 620) and determining whether the memory cell 232 has a resistance corresponding to the lower resistance state (step 630) until the resistance of the memory cell 532 corresponds to the lower resistance state, or until a predetermined number of retries are made. In some alternative embodiments the pulses of the retry bias arrangement may be changed for each iteration. If in step 630 it is determined that the memory cell has a resistance corresponding to the lower resistance state, the set operation terminates at step 650.

In the illustrated embodiment of FIG. 6, if the memory cell 532 has experienced a set failure the retry bias arrangement comprises the subsequent bias arrangement of step 640 combined with the first bias arrangement of step 620.

FIG. 10 illustrates a second embodiment of a set operation 1000 in which the retry (or second) bias arrangement 1040 does not include the first bias arrangement of step 620, and instead includes the function of setting the memory cell 532. FIG. 11 illustrates an embodiment of a timing diagram of the set operation 1000 of FIG. 10. As will be understood the timing diagram of FIG. 11 is simplified and not necessarily to scale.

Referring to FIGS. 11 and 12, in the illustrated embodiment the retry (or second) bias arrangement of step 1040 comprises applying a voltage V_(WL-RETRY) to word line 558, and applying a voltage pulse to the bit line 560 having a shape with initial pulse height of V_(HIGH) and a trailing edge in which the voltage on the bit line 560 decreases with time as shown. The voltage pulse is sufficient to breakthrough the high resistance layer and induces current to flow in path 580 and provide energy to the phase change material of memory element 548.

As represented heuristically in curve 1260 of FIG. 12, the pulse shape of the retry bias arrangement is adapted to breakthrough the high resistance layer and create a conduction path. Because of the trailing edge, the pulse shape is also adapted to cause at least a portion of the active region to transition into a crystalline phase, thereby establishing the lower resistance state. As will be understood, the curve 1260 is merely illustrative and the actual shape of the curve 1260 depends upon the properties of the memory cell, the manner in which the subsequent bias arrangement is applied to the memory cell, and the manner in which the phase change material heats up and cools down.

In some embodiments the amount of energy provided to the phase change material in step 1040 is sufficient to melt the active region of the phase change material and to cause a transition of at least an active region into a crystalline phase. In some alternative embodiments the energy provided to the phase change material in step 1040 is sufficient melt a portion of the phase change material greater than the active region, which may be useful for overcoming the set failure caused by compositional changes of the phase change material within the active region.

In the illustrated embodiment of FIGS. 10- 12, the retry bias arrangement of step 1040 comprises a single pulse as shown applied to the bit line 560, although it will be understood that other retry bias arrangements can alternatively be used. More generally, the retry bias arrangement of step 1040 may comprise a set of one or more pulses applied to the bit line 560 and/or word line 558 and/or source line 554 to induce current flow in path 580. The number of pulses and the pulse shapes, including the voltage levels and pulse times, of the subsequent bias arrangement can be determined empirically for each embodiment.

In the set operations 600, 1000 of FIGS. 6 and 10 the description refers to a single memory cell being programmed, although it will be understood that the set operations described herein are also applicable to programming a plurality of memory cells.

FIG. 13 illustrates an embodiment of set operation 1300 on a group of cells of array 405. In the following discussion the various bias arrangements can be implemented as described above including using pulses such as those described above, and thus a discussion of the various pulses and the bias arrangements of the set operation 1300 will not be repeated here.

The set operation 1300 for a group of memory cells of array 405 begins at step 1310. Step 1310 may include, or in some embodiments be preceded by, a read operation.

Next at step 1320 a first bias arrangement is applied to the group of memory cells to establish the lower resistance state in the memory cells.

At step 1330 the resistances of the memory cells are read to determine whether memory cells in the group of memory cells have respective resistances corresponding to the lower resistance state.

For memory cells in the group of memory cells not having a resistance corresponding to the lower resistance state, those memory cells have experienced a set failure and a retry (or second) bias arrangement is applied to those failed memory cells.

In the illustrated embodiment of FIG. 13, the retry bias arrangement of the set operation 1300 begins at step 1340 where a subsequent bias arrangement is applied to the memory cell.

Next, the retry bias arrangement of the set operation 1300 continues back to step 1320 where the first bias arrangement is applied to those failed memory cells.

The set operation 1300 then continues to step 1330 to determine whether the failed memory cells have a resistance corresponding to the lower resistance state. The set operation continues in the loop of iteratively applying the retry bias arrangement (steps 1340, 1320) to memory cells which again fail step 1330 and determining whether the failed memory cells from the preceding step 1330 have a resistance corresponding to the lower resistance state (step 1330) until the resistance of the memory cells corresponds to the lower resistance state, or until a maximum number of retries are made. If in step 1330 it is determined that each of the memory cells in the group has a resistance corresponding to the lower resistance state, the set operation terminates at step 1350.

In the set operation of FIG. 13, the retry bias arrangement comprises the subsequent bias arrangement of step 1340 combined with the first bias arrangement of step 1320. Alternatively, similar to the discussion above with respect to FIG. 10, the retry bias arrangement in some embodiments does not include the set bias arrangement of step 1320, and instead includes the function of setting the group of failed memory cells.

Memory devices and methods for operating such devices described herein address the set failure mode and result in improved endurance, reliability and improved data storage performance. Set operations described herein provide lower energy to the phase change material of memory cells to establish the lower resistance state and only apply higher energy to the phase change material when the lower energy is insufficient to set the memory cell. Thus, compared to melt-and-anneal set methods, set operations described herein reduce the amount of high current operations and thus improve the reliability of the memory cells.

Embodiments of the memory cells described herein include phase change based memory materials, including chalcogenide based materials and other materials, for the memory elements. Chalcogens include any of the four elements oxygen (O), sulfur (S), selenium (Se), and tellurium (Te), forming part of group VIA of the periodic table. Chalcogenides comprise compounds of a chalcogen with a more electropositive element or radical. Chalcogenide alloys comprise combinations of chalcogenides with other materials such as transition metals. A chalcogenide alloy usually contains one or more elements from group IVA of the periodic table of elements, such as germanium (Ge) and tin (Sn). Often, chalcogenide alloys include combinations including one or more of antimony (Sb), gallium (Ga), indium (In), and silver (Ag). Many phase change based memory materials have been described in technical literature, including alloys of: Ga/Sb, In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In the family of Ge/Sb/Te alloys, a wide range of alloy compositions may be workable. The compositions can be characterized as Te_(a)Ge_(b)Sb_(100-(a-b)). One researcher has described the most useful alloys as having an average concentration of Te in the deposited materials well below 70%, typically below about 60% and ranged in general from as low as about 23% up to about 58% Te and most preferably about 48% to 58% Te. Concentrations of Ge were above about 5% and ranged from a low of about 8% to about 30% average in the material, remaining generally below 50%. Most preferably, concentrations of Ge ranged from about 8% to about 40. The remainder of the principal constituent elements in this composition was Sb. These percentages are atomic percentages that total 100% of the atoms of the constituent elements. (Ovshinsky U.S. Pat. No. 5,687,112 patent, cols. 10-11.) Particular alloys evaluated by another researcher include Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7 (Noboru Yamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks for High-Data-Rate Recording”, SPIE v.3109, pp. 28-37 (1997).) More generally, a transition metal such as chromium (Cr), iron (Fe), nickel (Ni), niobium (Nb), palladium (Pd), platinum (Pt) and mixtures or alloys thereof may be combined with Ge/Sb/Te to form a phase change alloy that has programmable resistive properties. Specific examples of memory materials that may be useful are given in Ovshinsky '112 at columns 11-13, which examples are hereby incorporated by reference.

Chalcogenides and other phase change materials are doped with impurities in some embodiments to modify conductivity, transition temperature, melting temperature, and other properties of memory elements using the doped chalcogenides. Representative impurities used for doping chalcogenides include nitrogen, silicon, oxygen, silicon dioxide, silicon nitride, copper, silver, gold, aluminum, aluminum oxide, tantalum, tantalum oxide, tantalum nitride, titanium and titanium oxide. See, e.g., U.S. Pat. No. 6,800,504, and U.S. Patent Application Publication No. U.S. 2005/0029502.

Phase change alloys are capable of being switched between a first structural state in which the material is in a generally amorphous solid phase, and a second structural state in which the material is in a generally crystalline solid phase in its local order in the active channel region of the cell. These alloys are at least bistable. The term amorphous is used to refer to a relatively less ordered structure, more disordered than a single crystal, which has the detectable characteristics such as higher electrical resistivity than the crystalline phase. The term crystalline is used to refer to a relatively more ordered structure, more ordered than in an amorphous structure, which has detectable characteristics such as lower electrical resistivity than the amorphous phase. Typically, phase change materials may be electrically switched between different detectable states of local order across the spectrum between completely amorphous and completely crystalline states. Other material characteristics affected by the change between amorphous and crystalline phases include atomic order, free electron density and activation energy. The material may be switched either into different solid phases or into mixtures of two or more solid phases, providing a gray scale between completely amorphous and completely crystalline states. The electrical properties in the material may vary accordingly.

Phase change alloys can be changed from one phase state to another by application of electrical pulses. It has been observed that a shorter, higher amplitude pulse tends to change the phase change material to a generally amorphous state. A longer, lower amplitude pulse tends to change the phase change material to a generally crystalline state. The energy in a shorter, higher amplitude pulse is high enough to allow for bonds of the crystalline structure to be broken and short enough to prevent the atoms from realigning into a crystalline state. Appropriate profiles for pulses can be determined, without undue experimentation, specifically adapted to a particular phase change alloy. In following sections of the disclosure, the phase change material is referred to as GST, and it will be understood that other types of phase change materials can be used. A material useful for implementation of a PCRAM described herein is Ge₂Sb₂Te₅.

An exemplary method for forming chalcogenide material uses chemical vapor deposition CVD such as that disclosed in US Publication No 2006/0172067 entitled “Chemical Vapor Deposition of Chalcogenide Materials”, which is incorporated by reference herein.

A post-deposition annealing treatment in a vacuum or in an N2 ambient is optionally performed to improve the crystallize state of chalcogenide material. The annealing temperature typically ranges from 100° C. to 400° C. with an anneal time of less than 30 minutes.

While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims. 

1. A method for operating a memory cell comprising phase change material and programmable to a plurality of resistance states including a high resistance state and a lower resistance state, the method comprising: applying a first bias arrangement to the memory cell to establish the lower resistance state, the first bias arrangement comprising a first voltage pulse; determining whether the memory cell is in the lower resistance state; and if the memory cell is not in the lower resistance state, then applying a second bias arrangement to the memory cell to establish the lower resistance state, the second bias arrangement comprising a second voltage pulse having a pulse height greater than that of the first voltage pulse.
 2. The method of claim 1, wherein the second bias arrangement is sufficient to breakthrough a high resistance layer within an electrical conduction path of the memory cell.
 3. The method of claim 1, further comprising: determining whether the memory cell is in the lower resistance state after applying the second bias arrangement to the memory cell; and if the memory cell is not in the lower resistance state after applying the second bias arrangement to the memory cell, iteratively applying subsequent bias arrangements to the memory cell and determining whether the memory cell is in the lower resistance state until the memory cell is in the lower resistance state or a predetermined number of retries are made, wherein the subsequent bias arrangements respectively comprise a corresponding voltage pulse having a pulse height greater than that of the first voltage pulse to establish the lower resistance state.
 4. The method of claim 3, wherein the applying subsequent bias arrangements to the memory cell comprises applying the second bias arrangement to the memory cell.
 5. The method of claim 1, wherein the determining whether the memory cell is in the lower resistance state comprises applying a read bias arrangement to the memory cell and detecting a current in the memory cell, the current in the memory cell corresponding to the resistance state of the memory cell.
 6. The method of claim 1, wherein: the memory cell further comprises an access device having a first terminal coupled a word line and a second terminal coupled to a bit line via the phase change material; the applying the first bias arrangement comprises applying a voltage to the word line and applying the first voltage pulse to the bit line to induce a first current through the phase change material; and the applying the second bias arrangement to the memory cell comprises applying a voltage to the word line and applying the second voltage pulse to the bit line to induce a second current through the phase change material.
 7. The method of claim 6, wherein the second current through the phase change material is less than the first current.
 8. The method of claim 1, wherein applying the second bias arrangement comprises: applying the second voltage pulse across the phase change material; and after applying the second voltage pulse, applying the first voltage pulse across the phase change material.
 9. The method of claim 1, wherein the second voltage pulse is sufficient to melt an active region of the phase change material.
 10. The method of claim 1, wherein the second voltage pulse has a pulse shape adapted to melt at least an active region of the phase change material and to cause a transition of at least a portion of the active region into a crystalline phase.
 11. A memory device comprising: a memory cell comprising phase change material and programmable to a plurality of resistance states including a high resistance state and at least one lower resistance state; and bias circuitry adapted to apply bias arrangements to the memory cell, the bias arrangements including: a first bias arrangement to establish the lower resistance state, the first bias arrangement comprising a first voltage pulse; a read bias arrangement to determine whether the memory cell is in the lower resistance state after the first bias arrangement; and a second bias arrangement to establish the lower resistance state if the memory cell is not in the lower resistance state after the first bias arrangement, the second bias arrangement comprising a second voltage pulse having a pulse height greater than that of the first voltage pulse.
 12. The memory device of claim 11, wherein the second bias arrangement is sufficient to breakthrough a high resistance layer within an electrical conduction path of the memory cell.
 13. The memory device of claim 11, wherein the bias arrangements further include: a read bias arrangement to determine whether the memory cell is in the lower resistance state after the second bias arrangement; and a subsequent bias arrangement to establish the lower resistance state if the memory cell is not in the lower resistance state after the second bias arrangement, the subsequent bias arrangement comprising a voltage pulse having a pulse height greater than that of the first voltage pulse.
 14. The memory device of claim 13, wherein the subsequent bias arrangement comprises the second bias arrangement.
 15. The memory device of claim 11, wherein: the memory cell further comprises an access device having a first terminal coupled to a word line and a second terminal coupled to a bit line via the phase change material; the first bias arrangement comprises a voltage applied to the word line and the first voltage pulse applied to the bit line to induce a first current through the phase change material; and the second bias arrangement comprises a voltage applied to the word line and the second voltage pulse applied to the bit line to induce a second current through the phase change material.
 16. The memory device of claim 15, wherein the second current through the phase change material is less than the first current.
 17. The memory device of claim 11, wherein the second bias arrangement comprises a plurality of voltage pulses.
 18. The memory device of claim 12, wherein the second bias arrangement comprises: the second voltage pulse across the phase change material; and after the second voltage pulse, the first voltage pulse across the phase change material.
 19. The memory device of claim 11, wherein the second voltage pulse is sufficient to melt an active region of the phase change material.
 20. The memory device of claim 11, wherein the second voltage pulse has a pulse shape adapted to melt at least an active region of the phase change material and to cause a transition of at least a portion of the active region into a crystalline phase. 